1. Field of the Invention
This invention relates to logic gates, digital electronics for signal processing, storage and computation, and methods of fabricating the same.
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by one or more reference numbers within brackets, e.g., [x]. A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)
Spin Transfer Torque (STT) devices and spin valves are well documented in the literature. A spin valve device comprises a first conductive magnetic layer (fixed layer) and a second conductive magnetic layer (free layer), separated by a spacer layer, wherein the fixed layer, free layer, and spacer layers produce a device that exhibits Giant Magneto Resistance (GMR). The orientation of the magnetic moments in the fixed layer are fixed or pinned while the orientation of the magnetic moments in the free layer are not fixed and are consequently free to rotate in response to magnetic fields.
According to the GMR effect, when the magnetic moments in the fixed layer and the free layer are aligned (or parallel and pointing in the same direction), the device exhibits much smaller resistance as compared to when the magnetic moments in the fixed layer and the free layer are not aligned (or are anti parallel and pointing in the opposite direction).
Accordingly, if the magnetization orientation in the free layer is initially aligned with the magnetization orientation in the fixed layer (due to the weak spin coupling provided by the spacer layer), a magnetic field passing near the free layer may re-orient the magnetization orientation of the free layer with respect to the magnetization orientation of the fixed layer, causing a large change in resistance of the spin valve due to the GMR effect, which is sensed by current passing between the fixed layer and the free layer. Such an implementation is used as a read head sensor for hard drives, for example [1].
The GMR effect is also exploited for Magnetic Random Access Memory (MRAM). In the early MRAM the free layer magnetization reorientation was performed by applying a magnetic field generated by current in a write line: the state of the memory element is read by sensing the resistance via the GMR effect. More recently the spin transfer torque (STT) effect is used to write the magnetization state of the free layer [2-4]. An STT device (STTD) is a three layer device comprising a first, fixed, ferromagnetic metallic layer and a free, second, metallic, ferromagnetic layer separated from the first by a thin conductor or an insulator thin enough to allow electron tunneling. A spin polarized current (a current comprising electrons having predominantly one spin orientation) will reorient the free layer to be parallel or anti parallel as the electrons flow from the fixed to the free layer or from the free to fixed layer, respectively. In this way the relative magnetization orientation of the two layers can be “written” and “read” by suitable current flow through the STT device. Large currents are used to write (STT effect); small currents to sense the resistance (GMR effect).
The first and second layers of the STT device do not require power to retain their relative magnetization orientations, and therefore such a memory implementation is non-volatile—i.e., the data or information remains stored when the power to the device is switched off.
However, conventional magnetic memories or devices (e.g., MRAM) that are used to replace solid state memories such as Dynamic Random Access Memory (DRAM), Static Random Access Memory (SRAM), and flash memory, are not capable of executing logic. Instead, conventional devices use the transistor (e.g., CMOS) to perform logic operations. Replacing some transistors in the logic device with a patterned magnetic material may reduce the device size and cause such a device to be faster and cheaper.
Thus, there is a need for magnetic devices to perform logic operation in addition to being able to store information in a non-volatile way. The present invention satisfies this need by extending the capability of magnetic devices and adding logic operation to magnetic memory elements.